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class="sidebar-link">从链表中删除元素</a></li><li class="sidebar-sub-header"><a href="/front/算法和数据结构/常见数据结构/05.html#使用链表实现栈" class="sidebar-link">使用链表实现栈</a></li><li class="sidebar-sub-header"><a href="/front/算法和数据结构/常见数据结构/05.html#使用链表实现队列" class="sidebar-link">使用链表实现队列</a></li><li class="sidebar-sub-header"><a href="/front/算法和数据结构/常见数据结构/05.html#链表真题练习" class="sidebar-link">链表真题练习</a></li><li class="sidebar-sub-header"><a href="/front/算法和数据结构/常见数据结构/05.html#链表和递归" class="sidebar-link">链表和递归</a></li><li class="sidebar-sub-header"><a href="/front/算法和数据结构/常见数据结构/05.html#递归再深入" class="sidebar-link">递归再深入</a></li><li class="sidebar-sub-header"><a href="/front/算法和数据结构/常见数据结构/05.html#翻转链表" class="sidebar-link">翻转链表</a></li></ul></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h2 id="队列简介"><a href="#队列简介" class="header-anchor">#</a> 队列简介</h2> <p>队列是一种线性结构，相比数组，队列对应的操作是数组的子集，只能从一端(队尾)添加元素从另一端(队首)取出元素。队列是一种先进先出（First in First Out简写FIFO）的数据结构。</p> <p>用我们实际生活的场景来举例的话：就比如我们在车站排队买票，前面的乘客买完票后依次从队首离开，后面的需要买票的乘客依次从队尾排队进入买票。
<img src="/assets/img/13.5d732bff.png" alt=""></p> <h2 id="队列的基本实现"><a href="#队列的基本实现" class="header-anchor">#</a> 队列的基本实现</h2> <p>对于队列来说我们主要实现以下方法：</p> <table><thead><tr><th>方法名</th> <th>含义</th></tr></thead> <tbody><tr><td>void enqueue(E)</td> <td>入队（向队列中添加元素）</td></tr> <tr><td>E dequeue()</td> <td>出队（从队列中取出元素）</td></tr> <tr><td>E getFront()</td> <td>查看队首元素</td></tr> <tr><td>E getSize()</td> <td>查看队列中一共有多少元素</td></tr> <tr><td>boolean isEmpty()</td> <td>队列是否为空</td></tr></tbody></table> <p>这里我们使用之前实现的数组类来实现队列这种数据结构，由于调用的都是之前数组类中的方法这里不再赘述，直接给出具体实现：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">interface</span> <span class="token class-name">Queue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
    <span class="token keyword">int</span> <span class="token function">getSize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">boolean</span> <span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">void</span> <span class="token function">enqueue</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">E</span> <span class="token function">dequeue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">E</span> <span class="token function">getFront</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ArrayQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">implements</span> <span class="token class-name">Queue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
    <span class="token keyword">private</span> <span class="token class-name">Array</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> array<span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token class-name">ArrayQueue</span><span class="token punctuation">(</span><span class="token keyword">int</span> capacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        array <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>capacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token class-name">ArrayQueue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        array <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">getSize</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> array<span class="token punctuation">.</span><span class="token function">getSize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> array<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">enqueue</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        array<span class="token punctuation">.</span><span class="token function">addLast</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">dequeue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> array<span class="token punctuation">.</span><span class="token function">removeFirst</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">getFront</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> array<span class="token punctuation">.</span><span class="token function">getFirst</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">StringBuilder</span> res <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;Queue:&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;front [&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> array<span class="token punctuation">.</span><span class="token function">getSize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>array<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 如果元素不是最后一个元素</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>i <span class="token operator">!=</span> array<span class="token punctuation">.</span><span class="token function">getSize</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span>
                res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;,&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;] tail&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> res<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> array<span class="token punctuation">.</span><span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>测试我们实现的方法：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Main</span> <span class="token punctuation">{</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token class-name">ArrayQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> queue <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            queue<span class="token punctuation">.</span><span class="token function">enqueue</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>queue<span class="token punctuation">)</span><span class="token punctuation">;</span>
            
            <span class="token comment">// 每插入三个元素就从队列中取出一个元素</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>i <span class="token operator">%</span> <span class="token number">3</span> <span class="token operator">==</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                queue<span class="token punctuation">.</span><span class="token function">dequeue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>queue<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span> 
</code></pre></div><h2 id="复杂度分析"><a href="#复杂度分析" class="header-anchor">#</a> 复杂度分析</h2> <table><thead><tr><th>方法名</th> <th>时间复杂度</th> <th>备注</th></tr></thead> <tbody><tr><td>void enqueue(E e)</td> <td>O(1)</td> <td>均摊时间复杂度</td></tr> <tr><td>E dequeue()</td> <td>O(n)</td> <td></td></tr> <tr><td>int getSize()</td> <td>O(1)</td> <td></td></tr> <tr><td>boolean isEmpty()</td> <td>O(1)</td> <td></td></tr> <tr><td>E getFront()</td> <td>O(1)</td> <td></td></tr></tbody></table> <p>出队操作的时间复杂度是O(n)级别的，这是因为上述队列我们是通过数组实现的，当我们出队的时候也就是 把数组中的第一个元素拿出的时候，数组中其余的元素都要向前挪一位，所以出队的时间复杂度是O(n)的。</p> <h2 id="循环队列"><a href="#循环队列" class="header-anchor">#</a> 循环队列</h2> <p>上面我们通过数组实现的队列在出队的时候时间复杂度是O(n)级别的，有没有办法让队列出对的时候时间复杂度也是O(1)呢？</p> <p>前面我们已经说过，导致出队时间复杂度是O(n)级别的原因是因为把数组中的第一个元素拿出后，数组中其余的元素都要向前挪一位。那我们可不可以不让其他元素往前移呢？</p> <p>我们可以使用一个变量来记录下队首记为front队尾记为tail，当队首元素出队时只需要让front向前移动一位指向新的队首即可。</p> <p><img src="/assets/img/14.26a14e53.png" alt=""></p> <p>当front==tail时表明队列是空的，当有一个元素入队，只需要tail往后挪一位即可，依次类推。当队首元素出队时，只需要front向后挪动即可，依次类推，这时出队的时间复杂度就变成了O(1)。当将tail移动到队尾的时候此时再有元素入队，由于队首有空出的位置，可以将tail移动到队首的位置进行元素添加，直到<code>(tail+1)%capacity == front</code>的时候表示对列满了，此时需要扩容(注：capacity表示数组的长度)。</p> <p><img src="/assets/img/15.a153d8e7.png" alt=""></p> <p>入队操作</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">enqueue</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 判断队列是否满了，满了的话进行扩容</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>tail <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length <span class="token operator">==</span> front<span class="token punctuation">)</span>
        <span class="token function">resize</span><span class="token punctuation">(</span><span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 向队尾添加元素</span>
    data<span class="token punctuation">[</span>tail<span class="token punctuation">]</span> <span class="token operator">=</span> e<span class="token punctuation">;</span>
    <span class="token comment">// 将tail向后移一位，因为是循环队列需要对data.length取余，构成循环</span>
    tail <span class="token operator">=</span> <span class="token punctuation">(</span>tail <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
    size<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>扩容操作</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">resize</span><span class="token punctuation">(</span><span class="token keyword">int</span> newCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// newCapacity + 1 有意识的浪费一个空间</span>
    <span class="token class-name">E</span><span class="token punctuation">[</span><span class="token punctuation">]</span> newData <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">E</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>newCapacity <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
        <span class="token comment">// newData[i]对应的应该是data[i+front]，对data.length取余是为了让队列循环起来</span>
        newData<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> data<span class="token punctuation">[</span><span class="token punctuation">(</span>i <span class="token operator">+</span> front<span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length<span class="token punctuation">]</span><span class="token punctuation">;</span>

    data <span class="token operator">=</span> newData<span class="token punctuation">;</span>
    front <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    tail <span class="token operator">=</span> size<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>如图简单举例说下扩容操作，当i的取值是[0, 1]时newData[i]对应的是data[i+front]位置的元素，当 i= 2时newData[i]的取值对应的是data[(i+front) % 4]的值，由于data是一个循环队列newData[i]对应的值应该是data[(i+front) % 4]位置的值。</p> <p><img src="/assets/img/16.fa4bb8ea.png" alt=""></p> <p>出队操作</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">dequeue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token string">&quot;Cannot dequeue from an empty queue.&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">// 获取队首元素</span>
    <span class="token class-name">E</span> ret <span class="token operator">=</span> data<span class="token punctuation">[</span>front<span class="token punctuation">]</span><span class="token punctuation">;</span>
    <span class="token comment">// 将队首位置的元素置空，便于垃圾回收</span>
    data<span class="token punctuation">[</span>front<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    
    <span class="token comment">// 移动front，因为是循环队列，所以需要对data.length取余</span>
    front <span class="token operator">=</span> <span class="token punctuation">(</span>front <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
  
    size<span class="token operator">--</span><span class="token punctuation">;</span>

    <span class="token comment">// 如果队列中的元素出队了容积的四分之一，这时队列应该进行缩容，注意：缩容的值不能是零</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>size <span class="token operator">==</span> <span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">4</span> <span class="token operator">&amp;&amp;</span> <span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">2</span> <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span>
        <span class="token function">resize</span><span class="token punctuation">(</span><span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>完整代码：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">LoopQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">implements</span> <span class="token class-name">Queue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
    <span class="token keyword">private</span> <span class="token class-name">E</span><span class="token punctuation">[</span><span class="token punctuation">]</span> data<span class="token punctuation">;</span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> front<span class="token punctuation">,</span> tail<span class="token punctuation">,</span> size<span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token class-name">LoopQueue</span><span class="token punctuation">(</span><span class="token keyword">int</span> capacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">// capacity + 1是为了有意识的浪费一个空间，防止队列满时tail == front</span>
        data <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">E</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>capacity <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
        front <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        tail <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        size <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token class-name">LoopQueue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> data<span class="token punctuation">.</span>length <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> front <span class="token operator">==</span> tail<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">getSize</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> size<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">enqueue</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">// 判断队列是否满了，满了话进行扩容</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>tail <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length <span class="token operator">==</span> front<span class="token punctuation">)</span>
            <span class="token function">resize</span><span class="token punctuation">(</span><span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        data<span class="token punctuation">[</span>tail<span class="token punctuation">]</span> <span class="token operator">=</span> e<span class="token punctuation">;</span>
        tail <span class="token operator">=</span> <span class="token punctuation">(</span>tail <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
        size<span class="token operator">++</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">dequeue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
            <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token string">&quot;Cannot dequeue from an empty queue.&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">E</span> ret <span class="token operator">=</span> data<span class="token punctuation">[</span>front<span class="token punctuation">]</span><span class="token punctuation">;</span>
        data<span class="token punctuation">[</span>front<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        front <span class="token operator">=</span> <span class="token punctuation">(</span>front <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
        size<span class="token operator">--</span><span class="token punctuation">;</span>

        <span class="token keyword">if</span> <span class="token punctuation">(</span>size <span class="token operator">==</span> <span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">4</span> <span class="token operator">&amp;&amp;</span> <span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">2</span> <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span>
            <span class="token function">resize</span><span class="token punctuation">(</span><span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">getFront</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
            <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token string">&quot;Cannot dequeue from an empty queue.&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">return</span> data<span class="token punctuation">[</span>front<span class="token punctuation">]</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">resize</span><span class="token punctuation">(</span><span class="token keyword">int</span> newCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">E</span><span class="token punctuation">[</span><span class="token punctuation">]</span> newData <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">E</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>newCapacity <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
            newData<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> data<span class="token punctuation">[</span><span class="token punctuation">(</span>i <span class="token operator">+</span> front<span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length<span class="token punctuation">]</span><span class="token punctuation">;</span>

        data <span class="token operator">=</span> newData<span class="token punctuation">;</span>
        front <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        tail <span class="token operator">=</span> size<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">StringBuilder</span> res <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">format</span><span class="token punctuation">(</span><span class="token string">&quot;Queue: size = %d, capacity = %d\n&quot;</span><span class="token punctuation">,</span> size<span class="token punctuation">,</span> <span class="token function">getCapacity</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;front [&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">// 循环队列中第一个元素在front位置</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> front<span class="token punctuation">;</span> i <span class="token operator">!=</span> tail<span class="token punctuation">;</span> i <span class="token operator">=</span> <span class="token punctuation">(</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>data<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 不是队列中的最后一个元素</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> data<span class="token punctuation">.</span>length <span class="token operator">!=</span> tail<span class="token punctuation">)</span>
                res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;, &quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        res<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;] tail&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> res<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token class-name">LoopQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> queue <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">LoopQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            queue<span class="token punctuation">.</span><span class="token function">enqueue</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>

            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>queue<span class="token punctuation">)</span><span class="token punctuation">;</span>

            <span class="token keyword">if</span> <span class="token punctuation">(</span>i <span class="token operator">%</span> <span class="token number">3</span> <span class="token operator">==</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                queue<span class="token punctuation">.</span><span class="token function">dequeue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>queue<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="复杂度分析-2"><a href="#复杂度分析-2" class="header-anchor">#</a> 复杂度分析</h2> <table><thead><tr><th>方法名</th> <th>时间复杂度</th> <th>备注</th></tr></thead> <tbody><tr><td>void enqueue(E e)</td> <td>O(1)</td> <td>均摊时间复杂度</td></tr> <tr><td>E dequeue()</td> <td>O(1)</td> <td>均摊时间复杂度</td></tr> <tr><td>int getSize()</td> <td>O(1)</td> <td></td></tr> <tr><td>boolean isEmpty()</td> <td>O(1)</td> <td></td></tr> <tr><td>E getFront()</td> <td>O(1)</td> <td></td></tr></tbody></table> <h2 id="数组队列和循环队列的比较"><a href="#数组队列和循环队列的比较" class="header-anchor">#</a> 数组队列和循环队列的比较</h2> <p>上面我们费了半天劲终于实现了循环队列，那它比咱们之前实现队列的方法性能真的有提高吗？下面我们来编码测试下：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span></span><span class="token class-name">Random</span><span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Main</span> <span class="token punctuation">{</span>
    <span class="token comment">// 测试使用q运行opCount个enqueueu和dequeue操作所需要的时间，单位：秒</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">double</span> <span class="token function">testQueue</span><span class="token punctuation">(</span><span class="token class-name">Queue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> q<span class="token punctuation">,</span> <span class="token keyword">int</span> opCount<span class="token punctuation">)</span><span class="token punctuation">{</span>

        <span class="token keyword">long</span> startTime <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">Random</span> random <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span> <span class="token punctuation">;</span> i <span class="token operator">&lt;</span> opCount <span class="token punctuation">;</span> i <span class="token operator">++</span><span class="token punctuation">)</span>
            q<span class="token punctuation">.</span><span class="token function">enqueue</span><span class="token punctuation">(</span>random<span class="token punctuation">.</span><span class="token function">nextInt</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span>MAX_VALUE<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span> <span class="token punctuation">;</span> i <span class="token operator">&lt;</span> opCount <span class="token punctuation">;</span> i <span class="token operator">++</span><span class="token punctuation">)</span>
            q<span class="token punctuation">.</span><span class="token function">dequeue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">long</span> endTime <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">return</span> <span class="token punctuation">(</span>endTime <span class="token operator">-</span> startTime<span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">1000000000.0</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>

        <span class="token keyword">int</span> opCount <span class="token operator">=</span> <span class="token number">100000</span><span class="token punctuation">;</span>

        <span class="token class-name">ArrayQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> arrayQueue <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">double</span> time1 <span class="token operator">=</span> <span class="token function">testQueue</span><span class="token punctuation">(</span>arrayQueue<span class="token punctuation">,</span> opCount<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;ArrayQueue, time: &quot;</span> <span class="token operator">+</span> time1 <span class="token operator">+</span> <span class="token string">&quot; s&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">LoopQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> loopQueue <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">LoopQueue</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">double</span> time2 <span class="token operator">=</span> <span class="token function">testQueue</span><span class="token punctuation">(</span>loopQueue<span class="token punctuation">,</span> opCount<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;LoopQueue, time: &quot;</span> <span class="token operator">+</span> time2 <span class="token operator">+</span> <span class="token string">&quot; s&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>下图为运行上述测试代码结果截图：</p> <p><img src="/assets/img/17.5463a0c3.png" alt=""></p> <p>可以看到使用循环队列从性能上确实得到了不少的提升。</p> <p>使用size不浪费空间</p> <p>浪费一个空间不使用size</p> <h2 id="双端队列"><a href="#双端队列" class="header-anchor">#</a> 双端队列</h2> <p>可以在队列的两端添加元素，也可以在队列的两端删除元素。</p> <h2 id="练习"><a href="#练习" class="header-anchor">#</a> 练习</h2> <p>前面我们所实现的栈和队列都是通过动态数组来实现的，假设没有动态数组这种数据结构，现在有一个封装好的栈数据结构，我们怎么使用栈这种数据结构实现队列呢？反过来，给了一个封装好的队列这种数据结构，我们怎么使用这种数据结构实现栈呢？</p> <h3 id="第一题"><a href="#第一题" class="header-anchor">#</a> 第一题</h3> <p>请你仅使用两个队列实现一个后入先出（LIFO）的栈，并支持普通队列的全部四种操作（push、top、pop 和 empty）。</p> <p>实现 MyStack 类：</p> <ul><li>void push(int x) 将元素 x 压入栈顶。</li> <li>int pop() 移除并返回栈顶元素。</li> <li>int top() 返回栈顶元素。</li> <li>boolean empty() 如果栈是空的，返回 true ；否则，返回 false 。</li></ul> <p>注意：</p> <ul><li>你只能使用队列的基本操作 —— 也就是 push to back、peek/pop from front、size 和 is empty 这些操作。</li> <li>你所使用的语言也许不支持队列。 你可以使用 list （列表）或者 deque（双端队列）来模拟一个队列 , 只要是标准的队列操作即可。</li></ul> <p>示例：</p> <div class="language- extra-class"><pre class="language-text"><code>输入：
[&quot;MyStack&quot;, &quot;push&quot;, &quot;push&quot;, &quot;top&quot;, &quot;pop&quot;, &quot;empty&quot;]
[[], [1], [2], [], [], []]
输出：
[null, null, null, 2, 2, false]

解释：
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // 返回 2
myStack.pop(); // 返回 2
myStack.empty(); // 返回 False
</code></pre></div><p>提示：</p> <ul><li>1 &lt;= x &lt;= 9</li> <li>最多调用100 次 push、pop、top 和 empty</li> <li>每次调用 pop 和 top 都保证栈不为空</li></ul> <p>来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/implement-stack-using-queues
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。</p> <h3 id="第二题"><a href="#第二题" class="header-anchor">#</a> 第二题</h3> <p>请你仅使用两个栈实现先入先出队列。队列应当支持一般队列的支持的所有操作（push、pop、peek、empty）：</p> <p>实现 MyQueue 类：</p> <ul><li>void push(int x) 将元素 x 推到队列的末尾</li> <li>int pop() 从队列的开头移除并返回元素</li> <li>int peek() 返回队列开头的元素</li> <li>boolean empty() 如果队列为空，返回 true ；否则，返回 false</li></ul> <p>说明：</p> <ul><li>你只能使用标准的栈操作 —— 也就是只有 push to top, peek/pop from top, size, 和 is empty 操作是合法的。</li> <li>你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。</li></ul> <p>示例：</p> <div class="language- extra-class"><pre class="language-text"><code>输入：
[&quot;MyQueue&quot;, &quot;push&quot;, &quot;push&quot;, &quot;peek&quot;, &quot;pop&quot;, &quot;empty&quot;]
[[], [1], [2], [], [], []]
输出：
[null, null, null, 1, 1, false]

解释：
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false
</code></pre></div><p>提示：</p> <ul><li>1 &lt;= x &lt;= 9</li> <li>最多调用 100 次 push、pop、peek 和 empty</li> <li>假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）</li></ul> <p>来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/implement-queue-using-stacks
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。</p></div> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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